Hydrophilic member and method for manufacturing same

ABSTRACT

In a hydrophilic member including a structure in which a photocatalytic TiO 2  layer and a porous SiO 2  layer are stacked on a surface of a base material, easy forming of the porous SiO 2  layer so as to be thin and have a uniform film thickness distribution that enables the porous SiO 2  layer to cover an entire surface of the photocatalytic TiO 2  layer, and enhancement in durability of the porous SiO 2  layer are enabled. A photocatalytic TiO 2  layer is formed so as to have a density of 3.33 to 3.75 g/cm 3  (preferably 3.47 to 3.72 g/cm 3 , more preferably 3.54 to 3.68 g/cm 3 ) on a surface of a base material. As an outermost surface layer, a porous SiO 2  layer is formed on the photocatalytic TiO 2  layer in such a manner that the porous SiO 2  layer has a film thickness of no less than 10 nm and no more than 50 nm.

The disclosure of Japanese Patent Application No. JP2012-255257 filed on Nov. 21, 2012 including the specification, drawings, claims and abstract is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a hydrophilic member including a structure in which a TiO₂ (photocatalytic TiO₂) layer that provides photocatalysis and a porous SiO₂ layer are stacked on a surface of a base material, and a method for manufacturing the same. In particular, the present invention enables easy forming of the porous SiO₂ layer that is thin and has a uniform film thickness distribution that enables the porous SiO₂ layer to cover an entire surface of the photocatalytic TiO₂ layer, and enhancement in durability of the porous SiO₂ layer.

BACKGROUND ART

Examples of a hydrophilic member including a structure in which a photocatalytic TiO₂ layer and a porous SiO₂ layer are stacked on a surface of a base material are described in Patent Literatures 1 and 2. The hydrophilic member described in each of Patent Literatures 1 and 2 is one that ensures hydrophilicity by means of the porous SiO₂ layer at an outermost surface, decomposes organic matter and the like adhered to the porous SiO₂ layer by means of photocatalysis provided by the photocatalytic TiO₂ layer below the porous SiO₂ layer, and thereby enables hydrophilicity of the porous SiO₂ layer to be maintained for a long period of time.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Laid-Open No. 10-36144 -   Patent Literature 2: Japanese Patent Laid-Open No. 2000-53449

SUMMARY OF INVENTION Technical Problem

In a hydrophilic member including the aforementioned structure, in order to ensure uniform (that is, no irregularity through all regions of a hydrophilic surface of the hydrophilic member) and preferable hydrophilicity in every part of the hydrophilic surface, it is necessary to form a porous SiO₂ layer that is thin and has a uniform film thickness distribution that enables the porous SiO₂ layer to cover an entire surface of a photocatalytic TiO₂ layer when the film thickness of the porous SiO₂ layer is no more than 50 nm (preferably no more than 20 nm). However, it is not easy to form a porous SiO₂ layer that is thin and has a uniform film thickness distribution on a photocatalytic TiO₂ layer. In other words, in order to form a porous SiO₂ layer on a photocatalytic TiO₂ layer by means of, for example, vapor deposition, it is necessary to vapor-deposit SiO₂ with an increased gas pressure (partial pressure of oxygen gas) in a vapor deposition atmosphere compared to that for cases where a non-porous vapor-deposited SiO₂ layer is formed. However, vapor deposition with an increased gas pressure in a vapor deposition atmosphere causes instability in flight of SiO₂ vapor deposition molecules. Thus, irregularity in film thickness distribution occurs in some regions of the hydrophilic surface, resulting in the photocatalytic TiO₂ layer being partially exposed. Therefore, conventionally, in order to form an SiO₂ layer having a uniform film thickness distribution, some ingenuity (e.g., in arrangement of a correction plate and/or limiting the number of members to be subjected to the film forming process at a time) in the film forming process is required.

The present invention is intended to solve the aforementioned problems. In other words, the present invention is intended to provide a hydrophilic member and a method for manufacturing the same that enable easy forming of a porous SiO₂ layer that is thin and has a uniform film thickness distribution that enables the porous SiO₂ layer to cover an entire surface of a photocatalytic TiO₂ layer, thereby preventing the photocatalytic TiO₂ layer from being partially exposed, and enhancement in durability of the porous SiO₂ layer.

Solution to Problem

FIG. 2 indicates results of a test for measuring hydrophilicity recovery time of a hydrophilic member. The hydrophilic member used in this test is one formed by forming a photocatalytic TiO₂ layer on a surface of a flat and smooth base material and forming an SiO₂ layer having a film thickness of no more than 50 nm obtained by vapor-depositing SiO₂ vapor deposition molecules on the photocatalytic TiO₂ layer with a low gas pressure that enables stable flight of the SiO₂ vapor deposition molecules. Samples of the hydrophilic member with respective photocatalytic TiO₂ layers having different densities were prepared, and for each sample, time from a state in which hydrophilicity had been lost due to adherence of organic matter to a surface thereof to recovery of hydrophilicity due to ultraviolet irradiation (hydrophilicity recovery time) was measured. In this test, a surface of the SiO₂ layer of each sample was contaminated by an oil to lose hydrophilicity of the surface, and then, the surface is irradiated with an ultraviolet ray having an intensity of 1 mW/cm² using a black light. Recovery of hydrophilicity was determined when a water droplet contact angle was decreased to be comparable to an initial value before the contamination (no more than five degrees). Also, whether or not SiO₂ vapor deposition molecules stably fly during preparation of the samples can be determined based on, for example, whether or not a current (emission current) value of an electron beam or a vapor deposition speed during vapor deposition is stable. In this case, the vapor deposition speed can be measured as, for example, a derivative value of an oscillation frequency of a quartz-crystal film thickness meter. Also, the density of the photocatalytic TiO₂ layer of each sample can be adjusted by film forming conditions (e.g., a temperature of the base material, the film forming speed and/or the gas pressure), and the density can be measured by means of, for example, grazing incidence X-ray diffractometry. As can be seen from FIG. 2, as the density of the photocatalytic TiO₂ layer is lower, the hydrophilicity recovery time is shorter, and where the density exceeds 3.68 g/cm³, the hydrophilicity recovery time drastically increases, and where the density exceeds 3.75 g/cm³, the hydrophilicity recovery time becomes too long, the photocatalytic TiO₂ layer becomes impracticable. The hydrophilicity recovery time being short means that photocatalysis provided by the photocatalytic TiO₂ layer easily reaches the surface of the SiO₂ layer because the SiO₂ layer is porous. The hydrophilicity recovery time being long means that it is hard for photocatalysis provided by the photocatalytic TiO₂ layer to reach the surface of the SiO₂ layer because the SiO₂ layer is nonporous. The results of this test indicate that forming a photocatalytic TiO₂ layer so as to have a density of no more than 3.75 g/cm³ (preferably no more than 3.72 g/cm³, more preferably no more than 3.68 g/cm³) that is lower than 3.90 g/cm³, which is a typical density of anatase crystal structures, enables forming of a porous SiO₂ layer even if SiO₂ vapor deposition molecules is vapor-deposited on the photocatalytic TiO₂ layer with a low gas pressure that enables stable flight of the SiO₂ vapor deposition molecules. Since the vapor-deposition can be performed with a low gas pressure, a porous SiO₂ layer that is thin and has a uniform film thickness distribution can easily be formed with no special ingenuity in the film forming process. The test conducted by the present inventors indicates that where a photocatalytic TiO₂ layer having a density of no more than 3.75 g/cm³ is formed and SiO₂ vapor deposition molecules are vapor-deposited on the photocatalytic TiO₂ layer with a low gas pressure that enables stably flight of the SiO₂ vapor deposition molecules, a porous SiO₂ layer is formed. Also, if the porous SiO₂ layer has a thickness of no less than 10 nm, an entire surface of the photocatalytic TiO₂ layer can be covered by the porous SiO₂ layer (that is, partial exposure of the photocatalytic TiO₂ layer can be prevented).

FIG. 3 indicates results of a test in which for each of samples that are similar to those used in the test in FIG. 2 (samples of a hydrophilic member formed by forming a photocatalytic TiO₂ layer on a surface of a flat and smooth base material and forming an SiO₂ layer having a film thickness of no more than 50 nm obtained by vapor-depositing SiO₂ vapor deposition molecules on the photocatalytic TiO₂ layer with a low gas pressure that enables stable flight of the SiO₂ vapor deposition molecules, the samples including respective photocatalytic TiO₂ layers having different densities), a scratching load for the SiO₂ layer was measured. This test was conducted according to a procedure that is similar to that of a pencil hardness test, using an iron rod instead of a pencil, by measuring a load with each of weights having different weights employed. FIG. 3 indicates that as the density of the photocatalytic TiO₂ layer is lower, the SiO₂ layer formed on the photocatalytic TiO₂ layer is more brittle, and as the density of the photocatalytic TiO₂ layer is higher, the SiO₂ layer is harder.

FIG. 4 indicates results of a test in which for each of samples that are similar to those used in each of the tests in FIGS. 2 and 3, an acid-resistance of the SiO₂ layer was measured. This test was conducted by dropping H₂SO₄ having a concentration that is a normality of 0.1N on a surface of the SiO₂ layer and observing the state of the surface after being left for 24 hours. In this test, where the density of the photocatalytic TiO₂ layer is less than 3.33 g/cm³, a color of a part on which H₂SO₄ was dropped was more faded compared to a color of a part surrounding that part. This is because the base material was exposed as a result of the SiO₂ layer and the photocatalytic TiO₂ layer being stripped off at that part, resulting in no interference color generated by the SiO₂ layer and the photocatalytic TiO₂ layer. On the other hand, where the density of the photocatalytic TiO₂ layer is no less than 3.33 g/cm³, at the part on which H₂SO₄ was dropped, no fading occurred and the SiO₂ layer and the photocatalytic TiO₂ layer were not stripped off. Therefore, the test results in FIG. 4 indicate that where the density of the photocatalytic TiO₂ layer is less than 3.33 g/cm³, the acid-resistance is low and if the density of the photocatalytic TiO₂ layer is no less than 3.33 g/cm³, the acid-resistance is high.

The test results in FIGS. 3 and 4 indicate that forming a photocatalytic TiO₂ layer having a density of no less than 3.33 g/cm³ (preferably no less than 3.47 g/cm³, more preferably no less than 3.54 g/cm³) enables provision of a practical durability (scratch resistance and acid resistance).

Accordingly, the results of the tests in FIGS. 2 to 4 indicate that forming a photocatalytic TiO₂ layer having a density of 3.33 to 3.75 g/cm³ (preferably 3.47 to 3.72 g/cm³, more preferably 3.54 to 3.68 g/cm³) enables easy forming of a porous SiO₂ layer that is thin and has a uniform film thickness distribution that enables the porous SiO₂ layer to cover an entire surface of the photocatalytic TiO₂ layer and enhancement in durability of the porous SiO₂ layer.

Therefore, in the present invention, a photocatalytic TiO₂ layer having a density of 3.33 to 3.75 g/cm³ (preferably 3.47 to 3.72 g/cm³, more preferably 3.54 to 3.68 g/cm³) is formed on a surface of a base material and, as an outermost surface layer, a porous SiO₂ layer having a film thickness of no less than 10 nm and no more than 50 nm (preferably no less than 15 nm and no more than 20 nm) is formed on the TiO₂ layer in such a manner that the porous SiO₂ layer covers an entire surface of the TiO₂ layer. Consequently, a thin porous SiO₂ layer can be formed on a photocatalytic TiO₂ layer and can also be formed so as to have a uniform film thickness distribution that enables the porous SiO₂ layer to cover an entire surface of the photocatalytic TiO₂ layer, enabling provision of favorable and uniform photocatalysis by the photocatalytic TiO₂ layer. Also, the durability of the porous SiO₂ layer can be enhanced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional diagram illustrating an embodiment of a hydrophilic member of the present invention.

FIG. 2 is a diagram indicating results of a test in which for each of samples of a hydrophilic member (samples of a hydrophilic member formed by forming a photocatalytic TiO₂ layer on a surface of a flat and smooth base material and forming an SiO₂ layer having a film thickness of no more than 50 nm obtained by vapor-depositing SiO₂ vapor deposition molecules on the photocatalytic TiO₂ layer with a low gas pressure that enables stable flight of the SiO₂ vapor deposition molecules, the samples including respective photocatalytic TiO₂ layers having different densities), time from a state in which hydrophilicity had been lost due to adherence of organic matter to a surface thereof to recovery of hydrophilicity due to ultraviolet irradiation was measured.

FIG. 3 is a diagram indicating results of a test in which for each of samples that are similar to those used in the test in FIG. 2, a scratching load for the SiO₂ layer was measured.

FIG. 4 is a chart indicating results of a test in which for each of samples that are similar to those used in each of the tests in FIGS. 2 and 3, an acid-resistance of the SiO₂ layer was measured.

FIG. 5 is a schematic diagram illustrating an example of a vacuum vapor deposition apparatus 18 for manufacturing the hydrophilic member 10 in FIG. 1.

DESCRIPTION OF EMBODIMENT

FIG. 1 is a schematic cross-sectional diagram of an embodiment of a hydrophilic member of the present invention. A hydrophilic member 10 is configured by forming a photocatalytic TiO₂ layer 14 on a flat and smooth surface of a base material 12 and, as an outermost surface layer, forming a porous SiO₂ layer 16 on the photocatalytic TiO₂ layer 14. The porous SiO₂ layer 16 is formed so as to have a uniform film thickness distribution that enables the porous SiO₂ layer 16 to cover an entire surface of the photocatalytic TiO₂ layer. A density of the photocatalytic TiO₂ layer 14 is 3.33 to 3.75 g/cm³ (preferably 3.47 to 3.72 g/cm³, more preferably 3.54 to 3.68 g/cm³). A film thickness of the photocatalytic TiO₂ layer 14 is 50 to 500 nm. A film thickness of the porous SiO₂ layer 16 is no less than 10 nm and no more than 50 nm (preferably no less than 15 nm and no more than 25 nm).

The hydrophilic member 10 enables provision of, for example, an automobile window, or building window glass by forming the base material 12 using a transparent glass plate or a transparent resin plate. Also, the hydrophilic member 10 enables provision of, for example, a back surface mirror-type outer mirror for a vehicle or a back surface mirror such as a bathroom mirror by forming the base material 12 using a transparent glass plate or a transparent resin plate and forming a reflective film on a back surface of the base material 12. Also, the hydrophilic member 10 enables provision of, for example, a front surface mirror such as a front surface mirror-type outer mirror for an automobile by forming the base material 12 using a glass plate or a resin plate and forming a reflective film between the base material 12 and the photocatalytic TiO₂ layer 14. Also, the hydrophilic member 10 enables provision of a fog-resistant optical element by forming the base material 12 using an optical element such as a lens. If the base material 12 is a glass plate, a block layer (barrier layer) of, e.g., SiO₂ can additionally be arranged between the base material 12 and the photocatalytic TiO₂ layer 14 in order to prevent diffusion of alkali ions in the base material 12 into the photocatalytic TiO₂ layer 14.

An example of a method for manufacturing the hydrophilic member 10 in FIG. 1 will be described. Here, the base material 12 is formed using a glass plate, and each of the photocatalytic TiO₂ layer 14 and the porous SiO₂ layer 16 is formed by means of vapor deposition.

FIG. 5 illustrates an example of a vacuum vapor deposition apparatus 18. A vacuum chamber 20 is evacuated by a diffusion pump 22 and a rotary pump 24. In an upper portion of the vacuum chamber 20, a substrate holder 26 is arranged, and a glass plate 12, which forms a base material for the hydrophilic member 10, is held by the substrate holder 26, with a film-forming surface directed downward. The substrate holder 26 is heated by a heater 28 and the glass plate 12 is kept at a desired temperature via the substrate holder 26. A crucible 30 is arranged at a position below the glass plate 12, and a vapor deposition material (starting substance for vapor deposition) 32 is placed in the crucible 30. Examples of the vapor deposition material 32 for forming a TiO₂ layer 14 include, e.g., TiO₂, Ti₂O₃ and Ti. Examples of the vapor deposition material 32 for forming an SiO₂ layer 16 include, e.g., SiO₂ and SiO.

The vapor deposition material 32 is evaporated as a result of being irradiated with an electron beam 36 emitted from a hot cathode 34. As a reactive gas, an oxygen gas 42 is introduced from an oxygen tank 40 into the vacuum chamber 20. The evaporated vapor deposition material 32 reacts with the oxygen gas 42 to produce TiO₂ or SiO₂. The produced TiO₂ or SiO₂ is deposited on a surface of the glass plate 12, whereby a TiO₂ layer 14 or an SiO₂ layer 16 is formed. A film thickness during the film forming is monitored by a film thickness monitoring apparatus 44, and the vapor deposition is stopped when a desired film thickness is reached.

Film properties of the vapor-deposited film vary depending on, e.g., the temperature of the glass plate 12, the vapor deposition speed and the partial pressure of the oxygen gas 42 in the vacuum chamber 20. An example of film forming conditions for forming a photocatalytic TiO₂ layer having a density of 3.33 to 3.75 g/cm³ and forming a porous SiO₂ layer 16 on the photocatalytic TiO₂ layer 14 having a uniform film thickness distribution that enables the SiO₂ layer 16 to cover an entire surface of the photocatalytic TiO₂ layer if the film thickness of the porous SiO₂ layer 16 is no less than 10 nm is indicated in the following table.

Photocatalytic Porous SiO₂ TiO₂ layer 14 layer 16 Temperature of 300 degrees centigrade 300 degrees centigrade glass plate 12 Vapor 0.5 nm/sec. 0.2 nm/sec. deposition speed Partial pressure 0.016 Pa 0.016 Pa of oxygen gas 42

An example of a procedure for forming a photocatalytic TiO₂ layer 14 and a porous SiO₂ layer 16 using the vacuum vapor deposition apparatus 18 in FIG. 5 will be described below. A photocatalytic TiO₂ layer 14 is formed, for example, according to the following procedure.

-   (1) Hold a glass plate 12 in the substrate holder 26, place, for     example, Ti₂O₃ as a vapor deposition material 32 in the crucible 30,     and close the vacuum chamber 20. -   (2) Drive the rotary pump 24 and the diffusion pump 22 to evacuate     the vacuum chamber 20. -   (3) Drive the heater 28 to heat the glass plate 12 to a     predetermined temperature through the substrate holder 26. -   (4) Introduce an oxygen gas 42 from the oxygen tank 40 into the     vacuum chamber 20. -   (5) Drive the hot cathode 34 to irradiate the Ti₂O₃, which is a     vapor deposition material 32, with an electron beam 36 to evaporate     the Ti₂O₃. -   (6) The evaporated Ti₂O₃ reacts with the oxygen gas 42 to produce     TiO₂. The produced TiO₂ is deposited on the glass plate 12, whereby     a TiO₂ film is formed. -   (7) End the film forming when approximately 100 nm of TiO₂ is     deposited.

Upon the end of the forming of the photocatalytic TiO₂ layer 14, subsequently, a porous SiO₂ layer 16 is formed. A porous SiO₂ layer 16 is formed, for example, according to the following procedure.

-   (1) Place, for example, SiO₂ as a vapor deposition material 32 in     the crucible 30 and close the vacuum chamber 20. -   (2) Drive the rotary pump 24 and the diffusion pump 22 to evacuate     the vacuum chamber 20. -   (3) Drive the heater 28 to heat the glass plate 12 to a desired     temperature through the substrate holder 26. -   (4) Introduce an oxygen gas 42 from the oxygen tank 40 to the vacuum     chamber 20. -   (5) Drive the hot cathode 34 to irradiate the SiO₂, which is a vapor     deposition material 32, with an electron beam 36 to evaporate the     SiO₂. -   (6) The evaporated SiO₂ is deposited on the photocatalytic TiO₂     layer 14 on the glass plate 12, whereby a SiO₂ film is formed. -   (7) End the film forming when approximately 15 nm of SiO₂ is     deposited.

Since an outermost surface of the hydrophilic member 10 produced by the above process include the porous SiO₂ layer 16 alone, the hydrophilic member 10 exerts excellent effects in surface hardness and hydrophilicity maintenance compared to cases where the outermost surface includes a photocatalytic TiO₂ layer alone or a layer of a mixture of photocatalytic TiO₂ and SiO₂.

Although the above embodiment has been described in terms of a case where a photocatalytic TiO₂ layer and a porous SiO₂ layer are formed by means of vapor deposition, it can be considered that the effects of the invention according to the present application can also be expected where both or one of the layers is formed by means of another thin film forming method (for example, sputtering). 

The invention claimed is:
 1. A hydrophilic member comprising a structure in which a layer consisting of TiO₂ and having an anatase crystal structure and providing photocatalysis is formed so as to have a density of 3.33 to 3.75 g/cm³ on a surface of a base material, and a porous SiO₂ layer is formed as an outermost surface layer on the TiO₂ layer in such a manner that the porous SiO₂ layer has a thickness of no less than 10 nm and no more than 50 nm and covers an entire surface of the TiO₂ layer.
 2. The hydrophilic member according to claim 1, wherein the density of the TiO₂ layer is 3.47 to 3.72 g/cm³.
 3. The hydrophilic member according to claim 2, wherein the density of the TiO₂ layer is 3.54 to 3.68 g/cm³.
 4. The hydrophilic member according to claim 1, wherein the film thickness of the porous SiO₂ layer is no less than 15 nm and no more than 20 nm.
 5. The hydrophilic member according to claim 2, wherein the film thickness of the porous SiO₂ layer is no less than 15 nm and no more than 20 nm.
 6. The hydrophilic member according to claim 3, wherein the film thickness of the porous SiO₂ layer is no less than 15 nm and no more than 20 nm.
 7. A hydrophilic member manufacturing method comprising the steps of: forming a layer consisting of TiO₂ that provides photocatalysis, so as to have a density of 3.33 to 3.75 g/cm³ on a surface of a base material; and forming a porous SiO₂ layer as an outermost surface layer on the TiO₂ layer in such a manner that the porous SiO₂ layer has a thickness of no less than 10 nm and no more than 50 nm and covers an entire surface of the TiO₂ layer.
 8. The hydrophilic layer of claim 1, wherein the density of the TiO₂ layer is 3.33 to 3.54 g/cm³.
 9. A hydrophilic member comprising a structure in which a layer consisting of TiO₂ that provides photocatalysis is formed so as to have a density of 3.33 to 3.54 g/cm³ on a surface of a base material, and a porous SiO₂ layer is formed as an outermost surface layer on the TiO₂ layer in such a manner that the porous SiO₂ layer has a thickness of no less than 10 nm and no more than 50 nm and covers an entire surface of the TiO₂ layer. 